Dynamization of fracture fixation is used clinically to improve the bone healing process. However, the effect of early dynamization remains controversial. This study evaluated the effect of early dynamization, by reduced stiffness of fixation on callus stiffness and size after 5 weeks of healing in a rat diaphyseal femoral osteotomy. An external unilateral fixator allowed either a rigid (R-group; n = 8) or a flexible (F-group; n = 8) fixation. The dynamized group (D-group: n = 8) had a rigid fixation for 1 week, and then a flexible fixation for the remaining 4 weeks. The pre- and postoperative activity of the rats was measured. After 5 weeks, the rats were sacrificed, and healing was evaluated by biomechanical and densitometric methods. The R-group had a higher activity more closely approaching preoperative levels, compared to the D-group throughout all time points measured. This difference was significant after 14 days and 21 days. The flexural rigidity of the R-group was 82% (tested in the anterior-posterior direction; p = 0.01) and 93% (tested in the medial-lateral direction; p = 0.002) greater than the flexural rigidity of the D-group. The rigid fixation led to a stiffer callus with a smaller callus volume, but better mineralized tissue in the whole callus and at the level of the osteotomy gap than the flexible or the dynamized fixation. Early dynamization did not improve healing compared to rigid or flexible fixation in a rat femoral osteotomy model.